If the deck is in reasonably good condition, that is to say that it does not have a lot of gel coat cracks, then the process is simple but laborious. The surface may be chalky. That has to go. Wash thoroughly, sand the surface with 230 grit paper. Wet or dry type sandpaper, used wet, is the better plan. If you use wet or dry, put a few drops of dish detergent in a bucket of water. Use that water to keep the sanding surface wet. Rinse the sandpaper in the bucket frequently. What you are trying to do is prepare the surface by smoothing it and leaving a light matte finish on the part to be painted.
Rinse thoroughly with fresh water. Let the decks dry until no moisture is present. Inspect the surface for sanding residue, you will probably find some here and there. Wipe off the residue. A good epoxy primer is the next step. Follow instructions on the package. You can brush this on but you should not do it in the hot sunshine. find a cool shady place. Do not be stingy about buying a good quality brush for this job. Do not apply the primer too heavily, just try to do it smoothly.
When the primer has cured for a day or two you can proceed with the finish coat. Epoxy will stick to the fiberglass whereas many paints will not. Well not for long anyway. There is a problem with epoxy that you need to know about. The problem is that it does not endure UV radiation very well. It'll soon become chalky if exposed to a lot of sunshine. That is why you use a finish coat over the epoxy primer. Finish coat can be any good oil based paint. To be sure, some oil based paints are better than others but almost any name brand will do. If you do not mind the expense then get some so called boat paint from your friendly marine supply dealer. Those specialty paints are reliable but cost more than something that you might get at the hardware store or paint supply store. If you have a Sherwin Williams store in your area, then consult them. Their paint is not cheap but it sure does not cost as much as "marine grade" and it will do just as well.
If you do a decent job of this you can expect your paint to last several years. If you use epoxy primer you will need some epoxy solvent to clean that nice brush that you bought. You will need some solvent for the finish coat paint too. That will probably be simple mineral spirits. Epoxy uses butyl cellusolve or similar. Laquer thinner will work for cleaning but not as well as the epoxy solvent. After each solvent brush cleaning session, spend some time on your brush with soap and hot water. Use a coarse bristle scrub brush or a comb to work the soap into all the places that paint tends to hide inside that nice paint brush. Rinse carefully and hang it up to dry.
Good luck, and remember that these solvents are highly inflammable. Use caution. Do your painting in a well ventilated place or do it outdoors.